[simoncion]

> The angle of this that people may not realize is that San Francisco already owns the power generation (ie, the Hetch Hetchy dam).

My conclusion might be totally wrong because I'm converting from units that aren't meant to be converted, but...

If you filter down to only San Francisco County on this official-looking thingie [0], you see that it claims the city [1] has 5.126 TWh of consumption.

Your link [2] claims Hetch Hetchy Power System provides something like 395 MW of power generation capacity. I'm going to assume that that's a misprint and they meant to write MWh because that's the only thing that makes sense to me for a measure of power generation. While the last page of this PDF [3] indicates that the hydro generation component is capable of powering a bit more than twice the city's municipal demand, it seems like it's not enough to satisfy even 1/1000th of the demand of the entire city.

Perhaps I've totally fucked up my unit conversions (or relied on garbage data), but it looks like only the tiniest fraction of the city's power demands can be satisfied by the Hetch Hetchy dam. (Though, we could easily electrify way more of the Muni lines with the surplus capacity.)

[0] <https://www.energy.ca.gov/data-reports/energy-almanac/califo...>

[1] SF City and County are -AFAICT- the same thing. It's a little weird.

[2] <https://www.sfpuc.gov/about-us/our-systems/hetch-hetchy-powe...>

[3] <https://www.sfpuc.gov/sites/default/files/about-us/WeDeliver...> (found via [4]

[4] <https://www.sfpuc.gov/about-us/our-systems/storage-and-deliv...>

[DuckConference]

I would assume 395MW is the nameplate capacity, so you would multiply it by a capacity factor and time to get the energy production in a specific interval. Capacity factor in hydro can vary a lot by season and how much they want to produce vs reservoir levels, but for a back of the envelope 100% capacity factor you have 395 MW × 8,760 hours/year = 3.46TWh/year . Capacity factor could be in the high 90s in good conditions and maximum production but I expect it's a lot lower unless it's a wet year with very few big maintenance jobs needed.

[throwaway2037]

    > Capacity factor could be in the high 90s in good conditions and maximum production but I expect it's a lot lower unless it's a wet year with very few big maintenance jobs needed.

Most rivers this far away from the equator don't receive the same inflows each season. For example, the spring will have very high flow due to melting snow in the mountains that feed that reservoir. I guess the winter has the lowest flow because snow melts much less in the winter.

[hunterpayne]

I seriously doubt that any non-nuclear power generation in history has ever gotten to 90% capacity factor. Based on industry averages, HH is probably about 55-65% capacity. Things that effect this are: low demand periods (caused by lots of renewables), maintenance periods and low water levels at times (in winter) behind the dam.

Also, PG&E is HIGHLY regulated. Its prices, its generation and its executive compensation are all set by the state of CA. That SF wants to complain about those numbers is pretty amazing. Remember, they still don't have enough money in the tree trimming budget so that those same lines don't start fires in other parts of the state. Its the same wire that connects HH to SF. And somehow the city is going to manage that wire better?

PS The tree trimming budget is set by the state too. Why is it so low? Renewables cost money and that's one of the budgets they raided to get the funds.

[spullara]

It generates ~1 TWh per year or about 20% of SFs power. Power generation capacity is always in watts but it also doesn't run at 100% all the time.

[simoncion]

Ah. So I should have done something like

  395MW * 8760hr = 3,640,200 MWh

to figure out roughly the maximum possible annual demand that could be served by the system? (For the purposes of this question, I'm just doing the -er- dimensional/unit/whatever analysis and ignoring production-capacity-reducing factors that you'd definitely have to factor in to get the real number.)

If so, what a boneheaded, rookie mistake by me. Thanks for the reply.

[NewJazz]

Power generation facilities are typically quoted in terms of Watts, not Watt-hours. From an era where these were fossil fuel or nuclear plants or dams that provided a pretty steady level of energy. It indicates what the generation facility/asset can produce at any given moment, although if rivers run dry hydro output can decrease.

I think the numbers you cite in [0] are in terms of total annual consumption.

[hunterpayne]

The (nameplate) capacity is in watts. Its production is in watt hours. Some parties have intentionally confused these two concepts (capacity and production) to make certain power sources seem better than they are and other power sources worse than they are. The media consistently and clearly intentionally confuses these two concepts to prevent most people from learning how bad the numbers really are.

PS Capacity factor is the ratio between capacity and production. Its probably the single most important factor when comparing different generation types. Its intentionally made confusing because NPPs have 90+ cap factor and renewables have about 10ish cap factor. This makes renewables seem competitive when they are not. That's why this is always presented in a confusing manor. PPS Its also the main reason power is so expensive in CA.

[stephen_g]

It's not really anything to do with "an era where these [...] provided a pretty steady level of energy", it's just the only way that makes sense to describe it in any case.

Demand is always instantaneous, and transmission lines from a generator need to be sized for the maximum instantaneous generation, so in terms of the size of (any kind of) generator it's the main thing you're interested in. Capacity factor brings in seasonal / daily output variation but that's a whole different thing.

[jeffbee]

You did this wrong but drew the right conclusion. Hetch Hetchy system exists but it is nowhere near the scale of powering the entire city of San Francisco.

[simoncion]

Mmkay, so what were my core mistakes? Unit conversion failures? Order-of-magnitude errors? Sign problems (somehow)?

Both Wolfram Alpha and units(1) indicated that conversion between MW and MWh was totally nonsensical, so I presumed either that that was a typo, or that 1MW of power generation run for one hour will satisfy 1MWh of demand... assuming that either that demand is evenly spread throughout the hour, or that you can smooth over spikes with storage.

[hunterpayne]

Your mistake is ignoring capacity factor. Most power generation has between 40% and 60% capacity factor. NPPs are 90+ and renewables are about 10. This is intentionally made confusing and you aren't helping because you don't understand this. I do get that this is a good faith error caused by the intention conflation of different units by PR statements.